1. Field of Invention
This invention relates to rheometry, and, more specifically, to a method and apparatus for measuring fluid stresses in extension.
2. Related Art
Mobile polymer solutions, liquid-liquid emulsions, and dilute solid-in-liquid suspensions show flow behavior in which the observed stress has an origin that is partly viscous and partly elastic. These viscous and elastic properties can be measured using a variety of viscometers. Knowledge of these properties is needed for purposes of material characterization, product differentiation, product formulation, and quality control.
Many fluids of industrial interest show only low levels of elasticity, and researchers have found that the use of an extensional flow field is best suited for fluid elasticity measurement. However, extensional viscometers that can make measurement under flow conditions representative of industrial operations require large liquid volumes on the order of one quart. This is because flow has to be set up and a steady state established before the fluid stress can be measured. This is often problematic because sample quantities are often limited due to the expense of the liquid to be measured.
When limited samples are available, fluid recirculation is necessary. Fluid recirculation is undesirable because polymer chain scission happens easily during stretching. Furthermore, in applications such as aerosol formation during agricultural pesticide and herbicide spraying, and also in inkjet printing, the liquids of interest are relatively inelastic and have shear viscosities that are as low as 5 or 6 mPa·s. Measuring extensional viscosity of these liquids is extremely difficult. Therefore, there is a need for a method and apparatus capable of making fluid elasticity measurements with only small volumes of fluid and without the need for re-circulating the sample. There is a further need for a method and apparatus that can make these measurements for relatively inelastic fluids having low shear viscosities.